This invention is directed to star couplers and in particular to reflection and hybrid transmission-reflection star couplers for distributing light coupled into any one or more ports to all of the output ports in the couplers.
A transmission star coupler is a multi-port optical device which distributes the light coupled into an input port to all of the output ports of the device, such a device is described in U.S. patent application Ser. No. 805,865 (now U.S. Pat. No. 4,291,940), filed June 13, 1977 by B. S. Kawasaki et al.; and in the publication by E. G. Rawson et al., Electronics Letters, Vol. 15, No. 14, July 5, 1979, pp. 432-433. A reflection star coupler distributes the light coupled into any one port to all of the ports of the device. Thus the input ports of a reflection star are also the output ports.
A hybrid transmission-reflection star coupler is a new type star coupler in which some of the ports function as in a transmission star coupler and the rest of the ports function as in a reflection star coupler.
Reflection star couplers are useful for implementing passively coupled fiber-optic data bus networks which interconnect a large number of terminals or nodes. In such a data-bus network implementation, each terminal is connected by a single fiber strand to a port of the reflection star couplers. A low-loss access coupler or an optical combiner located at each terminal is required to permit bidirectional transmission over the single fiber strand. The general technique for fabricating reflection star couplers is to use a mixing rod and a mirror to distribute the light among the ports of the device as described in U.S. Pat. No. 3,874,781 which issued on Apr. 1, 1975 to F. L. Thiel. U.S. Pat. No. 4,092,059, which issued on May 30, 1978 to T. Hawkes et al., teaches a device in which the mirror is replaced by a fiber bundle loop. Recently, a reflection star has been described by T. Ito et al., Proceedings of the Fourth European Conference on Optical Communications, Geneva, Sept. 12-15, 1978, pp. 318-322, in which the mixing rod has been replaced by a fused fiber taper structure. These techniques are limited to some extent by large excess loss (2.1 to 4.5 dB) and their complex fabrication procedures which result in a high cost.